setns(2) - Linux manual page (original) (raw)
setns(2) System Calls Manual setns(2)
NAME top
setns - reassociate thread with a namespaceLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#define _GNU_SOURCE** /* See feature_test_macros(7) */
**#include <sched.h>**
**int setns(int** _fd_**, int** _nstype_**);**DESCRIPTION top
The **setns**() system call allows the calling thread to move into
different namespaces. The _fd_ argument is one of the following:
• a file descriptor referring to one of the magic links in a
_/proc/_pid_/ns/_ directory (or a bind mount to such a link);
• a PID file descriptor (see [pidfd_open(2)](../man2/pidfd%5Fopen.2.html)).
The _nstype_ argument is interpreted differently in each case.fd refers to a /proc/pid/ns/ link If fd refers to a /proc/pid/ns/ link, then setns() reassociates the calling thread with the namespace associated with that link, subject to any constraints imposed by the nstype argument. In this usage, each call to setns() changes just one of the caller's namespace memberships.
The _nstype_ argument specifies which type of namespace the calling
thread may be reassociated with. This argument can have _one_ of
the following values:
**0** Allow any type of namespace to be joined.
**CLONE_NEWCGROUP** (since Linux 4.6)
_fd_ must refer to a cgroup namespace.
**CLONE_NEWIPC** (since Linux 3.0)
_fd_ must refer to an IPC namespace.
**CLONE_NEWNET** (since Linux 3.0)
_fd_ must refer to a network namespace.
**CLONE_NEWNS** (since Linux 3.8)
_fd_ must refer to a mount namespace.
**CLONE_NEWPID** (since Linux 3.8)
_fd_ must refer to a descendant PID namespace.
**CLONE_NEWTIME** (since Linux 5.8)
_fd_ must refer to a time namespace.
**CLONE_NEWUSER** (since Linux 3.8)
_fd_ must refer to a user namespace.
**CLONE_NEWUTS** (since Linux 3.0)
_fd_ must refer to a UTS namespace.
Specifying _nstype_ as 0 suffices if the caller knows (or does not
care) what type of namespace is referred to by _fd_. Specifying a
nonzero value for _nstype_ is useful if the caller does not know
what type of namespace is referred to by _fd_ and wants to ensure
that the namespace is of a particular type. (The caller might not
know the type of the namespace referred to by _fd_ if the file
descriptor was opened by another process and, for example, passed
to the caller via a UNIX domain socket.)fd is a PID file descriptor Since Linux 5.8, fd may refer to a PID file descriptor obtained from pidfd_open(2) or clone(2). In this usage, setns() atomically moves the calling thread into one or more of the same namespaces as the thread referred to by fd.
The _nstype_ argument is a bit mask specified by ORing together _one_
_or more_ of the **CLONE_NEW*** namespace constants listed above. The
caller is moved into each of the target thread's namespaces that
is specified in _nstype_; the caller's memberships in the remaining
namespaces are left unchanged.
For example, the following code would move the caller into the
same user, network, and UTS namespaces as PID 1234, but would
leave the caller's other namespace memberships unchanged:
int fd = pidfd_open(1234, 0);
setns(fd, CLONE_NEWUSER | CLONE_NEWNET | CLONE_NEWUTS);Details for specific namespace types Note the following details and restrictions when reassociating with specific namespace types:
User namespaces
A process reassociating itself with a user namespace must
have the **CAP_SYS_ADMIN** capability in the target user
namespace. (This necessarily implies that it is only
possible to join a descendant user namespace.) Upon
successfully joining a user namespace, a process is granted
all capabilities in that namespace, regardless of its user
and group IDs.
A multithreaded process may not change user namespace with
**setns**().
It is not permitted to use **setns**() to reenter the caller's
current user namespace. This prevents a caller that has
dropped capabilities from regaining those capabilities via
a call to **setns**().
For security reasons, a process can't join a new user
namespace if it is sharing filesystem-related attributes
(the attributes whose sharing is controlled by the [clone(2)](../man2/clone.2.html)
**CLONE_FS** flag) with another process.
For further details on user namespaces, see
[user_namespaces(7)](../man7/user%5Fnamespaces.7.html).
Mount namespaces
Changing the mount namespace requires that the caller
possess both **CAP_SYS_CHROOT** and **CAP_SYS_ADMIN** capabilities
in its own user namespace and **CAP_SYS_ADMIN** in the user
namespace that owns the target mount namespace.
A process can't join a new mount namespace if it is sharing
filesystem-related attributes (the attributes whose sharing
is controlled by the [clone(2)](../man2/clone.2.html) **CLONE_FS** flag) with another
process.
See [user_namespaces(7)](../man7/user%5Fnamespaces.7.html) for details on the interaction of
user namespaces and mount namespaces.
PID namespaces
In order to reassociate itself with a new PID namespace,
the caller must have the **CAP_SYS_ADMIN** capability both in
its own user namespace and in the user namespace that owns
the target PID namespace.
Reassociating the PID namespace has somewhat different from
other namespace types. Reassociating the calling thread
with a PID namespace changes only the PID namespace that
subsequently created child processes of the caller will be
placed in; it does not change the PID namespace of the
caller itself.
Reassociating with a PID namespace is allowed only if the
target PID namespace is a descendant (child, grandchild,
etc.) of, or is the same as, the current PID namespace of
the caller.
For further details on PID namespaces, see
[pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html).
Cgroup namespaces
In order to reassociate itself with a new cgroup namespace,
the caller must have the **CAP_SYS_ADMIN** capability both in
its own user namespace and in the user namespace that owns
the target cgroup namespace.
Using **setns**() to change the caller's cgroup namespace does
not change the caller's cgroup memberships.
Time namespaces
In order to reassociate itself with a new time namespace,
the caller must have the **CAP_SYS_ADMIN** capability both in
its own user namespace and in the user namespace that owns
the target namespace.
A multithreaded process may not change time namespace with
**setns**().
Network, IPC, and UTS namespaces
In order to reassociate itself with a new network, IPC,
time, or UTS namespace, the caller must have the
**CAP_SYS_ADMIN** capability both in its own user namespace and
in the user namespace that owns the target namespace.RETURN VALUE top
On success, **setns**() returns 0. On failure, -1 is returned and
_[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**EBADF** _fd_ is not a valid file descriptor.
**EINVAL** _fd_ refers to a namespace whose type does not match that
specified in _nstype_.
**EINVAL** There is problem with reassociating the thread with the
specified namespace.
**EINVAL** The caller tried to join an ancestor (parent, grandparent,
and so on) PID namespace.
**EINVAL** The caller attempted to join the user namespace in which it
is already a member.
**EINVAL** The caller shares filesystem (**CLONE_FS**) state (in
particular, the root directory) with other processes and
tried to join a new user namespace.
**EINVAL** The caller is multithreaded and tried to join a new user
namespace.
**EINVAL** _fd_ is a PID file descriptor and _nstype_ is invalid (e.g., it
is 0).
**ENOMEM** Cannot allocate sufficient memory to change the specified
namespace.
**EPERM** The calling thread did not have the required capability for
this operation.
**ESRCH** _fd_ is a PID file descriptor but the process it refers to no
longer exists (i.e., it has terminated and been waited on).STANDARDS top
Linux.VERSIONS top
Linux 3.0, glibc 2.14.NOTES top
For further information on the _/proc/_pid_/ns/_ magic links, see
[namespaces(7)](../man7/namespaces.7.html).
Not all of the attributes that can be shared when a new thread is
created using [clone(2)](../man2/clone.2.html) can be changed using **setns**().EXAMPLES top
The program below takes two or more arguments. The first argument
specifies the pathname of a namespace file in an existing
_/proc/_pid_/ns/_ directory. The remaining arguments specify a
command and its arguments. The program opens the namespace file,
joins that namespace using **setns**(), and executes the specified
command inside that namespace.
The following shell session demonstrates the use of this program
(compiled as a binary named _nsexec_) in conjunction with the
**CLONE_NEWUTS** example program in the [clone(2)](../man2/clone.2.html) man page (complied as
a binary named _newuts_).
We begin by executing the example program in [clone(2)](../man2/clone.2.html) in the
background. That program creates a child in a separate UTS
namespace. The child changes the hostname in its namespace, and
then both processes display the hostnames in their UTS namespaces,
so that we can see that they are different.
$ **su** # Need privilege for namespace operations
Password:
# **./newuts bizarro &**
[1] 3549
clone() returned 3550
uts.nodename in child: bizarro
uts.nodename in parent: antero
# **uname -n** # Verify hostname in the shell
antero
We then run the program shown below, using it to execute a shell.
Inside that shell, we verify that the hostname is the one set by
the child created by the first program:
# **./ns_exec /proc/3550/ns/uts /bin/bash**
# **uname -n** # Executed in shell started by ns_exec
bizarroProgram source #define _GNU_SOURCE #include <err.h> #include <fcntl.h> #include <sched.h> #include <stdio.h> #include <stdlib.h> #include <unistd.h>
int
main(int argc, char *argv[])
{
int fd;
if (argc < 3) {
fprintf(stderr, "%s /proc/PID/ns/FILE cmd args...\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Get file descriptor for namespace; the file descriptor is opened
with O_CLOEXEC so as to ensure that it is not inherited by the
program that is later executed. */
fd = open(argv[1], O_RDONLY | O_CLOEXEC);
if (fd == -1)
err(EXIT_FAILURE, "open");
if (setns(fd, 0) == -1) /* Join that namespace */
err(EXIT_FAILURE, "setns");
execvp(argv[2], &argv[2]); /* Execute a command in namespace */
err(EXIT_FAILURE, "execvp");
}SEE ALSO top
[nsenter(1)](../man1/nsenter.1.html), [clone(2)](../man2/clone.2.html), [fork(2)](../man2/fork.2.html), [unshare(2)](../man2/unshare.2.html), [vfork(2)](../man2/vfork.2.html),
[namespaces(7)](../man7/namespaces.7.html), [unix(7)](../man7/unix.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-07 setns(2)
Pages that refer to this page:nsenter(1), clone(2), pidfd_open(2), syscalls(2), unshare(2), proc_pid_ns(5), systemd.exec(5), capabilities(7), cgroup_namespaces(7), ipc_namespaces(7), mount_namespaces(7), namespaces(7), pid_namespaces(7), time_namespaces(7), user_namespaces(7), uts_namespaces(7), ip-netns(8)